During the drilling and completion stage of an oil or gas producing well, it is customary to introduce a metal pipe referred to as a casing into the hole being drilled to create an annular space between the metal pipe and the open hole representing the formation being drilled. As the drilling operation proceeds, the casing size is reduced in two or more deliberate steps so that the surface casing is the largest diameter and the final casing in the producing intervals is the smallest diameter. To fill the void between the outermost casing wall and the boundary of the drilled hole, it is routine practice to flow a sufficient volume of cement slurry down the casing and return it back up the annular space between the casing and the formation to completely fill the annular space with cement slurry. When hardened, the cemented annulus provides a cement column which serves to support and localize the metal casing, protects the casing from corrosion, and most significantly, seals the annulus from fluid flow between producing intervals, and between a producing interval and the surface.
Prior to the completion of the hardening process, the cement goes through a number of distinct steps including the initial placement of the cement slurry, the gelation or transition state of the slurry, and then the final set condition of the cement. During the gelation step the volume of the cement decreases slightly. The combination of gelation and shrinkage causes a decline in the hydrostatic pressure exerted by the cement column. This loss of hydrostatic head allows the influx of gas from permeable formations into the still gelling cement forming channels for gas to migrate between formation zones or between a zone and the surface. i,e a gas migration problem.
Another undesirable effect of this loss of hydrostatic head is the separation of the cement bond from the casing and/or the formation. This lost bonding also causes a gas migration problem by providing another mechanism for communication between formation zones through the annulus As a consequence of these various mechanisms, vertical fractures and channels develop in the setting cement that allow for inter-zone fluid migration and fluid migration between producing zones and the surface. No gas migration through or around the cement column is acceptable because inter-zone gas communication can lead to significant loss of hydrocarbons to non-producing formations. In addition, gas migration to the surface can result in a dangerous condition that could cause a loss of the producing well.
In addition to the fractures and channels problem, because no cement mix can be viewed as being truly impermeable in the final hardened form, there is always some inherent residual permeability in the cement column. Although gas migration due to this residual permeability in the cement column can be expected to be significantly lower than the gas migration observed when there are fine fracture paths in the column, it can present gas migration problems sufficient to warrant attempting to address the problem. Singly or in combination, such migration can lead to a condition referred to as excess or positive casing pressure, i.e. pressure on the casing increases due to this fluid influx. The positive casing pressure must be released or relieved before the pressure causes casing collapse.
A number of procedures have been explored to mitigate the circumstances that lead to the undesirable migration paths in and around cement sheaths. The earliest approaches to preventing paths during the cementing process involved physically jarring the casing to help with the settling of the cement to minimize volume losses during the shrinking stage. Another early preventative approach involved injecting pressurized water into the annulus at the surface to attempt to restore lost hydrostatic head during the cement gelation process. Yet another approach involved the direct vibration of the cement using pressure pulses generated by a water pulse generator. A more recent approach replaces the water pulses with air pulses. Cement formulations are also available with special ingredients added in an attempt to minimize the volume shrinkage during the gelling phase.
Despite these efforts to eliminate or minimize channels in and around cement sheaths, thousands of completed gas and oil wells have flawed cement sheaths. In the Gulf of Mexico alone there are thought to be between 8,000 and 11,000 wells displaying a problem of excess casing pressure that needs to be remedied. For underwater wells such as those located in the Gulf of Mexico or in the North Sea off the coast of Great Britain or Norway, casing pressure due to gas build up is particularly problematic due to heightened environmental and safety concerns.
Consequently, there persists a need for a post-cementing remedial step that will address the channels responsible for the gas migration problem. Classically, the remedial step has been a cement squeeze where very fine grained cement is squeezed into the wellbore region with the expectation that this cement slurry will penetrate the offending channels and shut off the gas flow. Apart from the fact that such a cement squeeze is quite expensive, the particle size of the slurry which is being injected limits its ability to penetrate deep into the offending channels. Adding to the problem is the high density of the cement slurry which hinders its vertical mobility. Accordingly, there remains a compelling need to develop a technology that will easily and in-depth penetrate the bulk of the channels that have formed and then effectively plug them off.
My earlier patent, U.S. Pat. No. 5,095,984 offers a unique mechanism for in-depth delivery of a plugging agent to a high permeability thief zone in a formation using a compressed gas phase. This patent, incorporated herein by reference, basically teaches a method of delivering a combination of compressed gas, cosolvent and polymer or surfactant that has been adjusted to be one phase at some specific temperature and pressure conditions, as defined by some specific application or reservoir properties, to a formation in a form that will plug an oil bearing formation if the temperature of the original mixture is raised or the pressure lowered from the conditions where the system has been made one phase. The present invention uses that basic concept to address the problem of gas migration into and through cement sheaths.